The two major aims of this study are to: (1) Describe and compare the histopathologic characteristics; serum biochemical features; and condition- specific clinical performance, functional status, and health perceptions of patients who have developed myopathy during antiretroviral therapy for HIV infection; and (2) Contrast the changes in histopathologic characteristics; serum biochemical features; and myopathy-related clinical performance, functional status, and health perceptions that occur over time, and following clinically-determined interventions such as altered antiretroviral doses, steroid administration, or nonsteroidal anti- inflammatory medications. In addition, it explores differences in responses to general and newly developed condition-specific items of an adapted version of the Short Form of the Medical Outcomes Study questionnaire (SF-36 MOS) in order to determine health perceptions and health status as judged by the patient. A concurrent, prospective case series design is used which involves longitudinal measurement of most study variables at monthly intervals for the original six month duration of the study, with the exception of muscle biopsy which is performed at the beginning and then repeated once. One additional evaluation at around 12 months has been added. 25 evaluable subjects have been recruited, involving consecutive patients participating in NIAID HIV treatment trials who meet the study criteria. Laboratory studies performed at baseline and monthly for six months include: CBC with differential; Acute Care, Hepatic, and Mineral Panels; CK; LDH; Aldolase; Lactate and Urinalysis. Clinical performance is evaluated at baseline and monthly for six months by manual muscle strength testing, while functional status is assessed by timed muscle activities and functional grade. Health perceptions and health status are measured by the MOS questionnaire.